Even if that was rude, I agree with #3 here, just because a girl wants to break up doesn't make her a bitch, but writing 13 pages about it to a guy who just wrote a whole song for her makes her a bitch for sure.
Breaking up is already a painful experience, unless you're asked why, blabbing about it to the other person isn't needed.

Unless OP is a horrible lover, and it doesn't seem like it, she could have just met up with him and told him it wasn't going to work.

While 13 pages does seem erm, a tad excessive, I think it's much more thoughtful to try and explain why you're breaking up with someone, rather than just dumping someone like that. I think it's sort of sweet she took the time and effort of writing a letter, although it does seem like she got carried away a bit! And I'm thinking she'd probably already written the letter, you don't just write a 13 page letter on the spot, so I'd be surprised if it was written as a reaction to the love song...

Yeah, this. I get the feeling that if those 13 pages were entirely unreasonable, trivial shit, that would've been mentioned in the FML too. Writing a song doesn't negate it if he treated her shittily beforehand.

It sounds like she didn't leave anything unsaid. But really if I had to guess, I'd say she'd have her points followed by specific examples so he couldn't just deny to her or himself that he had the quality or took that action or whatever the point was. Or she had complicated personal issues that needed resolved outside of him. Maybe both, it was 13 pages.